Charge density waves in layered materials

Charge density waves (CDW) are periodic modulations of the charge density in low-dimensional metals, which can be also characterized by periodic lattice distortion (PLD) which results in a modulation of atomic positions. Quasi two-dimensional (2D) transition metal dichalcogenides including 1T/2H-TaSe2, 1T/2H-TaS2, 2H-NbSe2, and 1T-TiSe2 are some of the low dimensional metals that exhibit strong CDW distortions whose transition temperature and commensuration varies mainly with temperature. In dependence of the dimensionality, pressure, intercalation, and doping, the transition temperature can also be affected. Thus, structural studies using electron diffraction, atomically resolved imaging (see Fig. 1), and electron energy-loss spectroscopy of 2D transition metal dichalcogenides that exhibit charge density waves/periodic lattice distortions. This includes 1T/2H-TaSe2, 1T/2H-TaS2 and 1T-TiSe2

Moreover, transition-metal dichalcogenides can be intercalated with alkali metals and many organic molecules. This may result in dramatic changes in the atomic and electronic structure as well as transport properties of the host structure. We are conducting systematic intercalation studies of CDW materials with organic molecules and study changes in the electronic and atomic structure.

Figure 1: (a) Selected area diffraction pattern of 1T-TaSe2 at 100 K(b) HRTEM image of 1T-TaSe2 at 100 K showing the atomic column intensity modulation distribution due PLD . The HRTEM image in 2(b) shows intensity modulation due to the commensurate CDW/PLD. The PLD superlattice consists of 4 clusters of 13 Ta atom having Ta-Ta distorted interatomic distances of down to 0.4 Å.

Collaborations

Prof. Mika Linden, Ulm University

Projects / Funding

DFG and MWK Baden-Württemberg in the frame of the SALVE project